1. Field of the Invention
This invention relates to an internal combustion engine introducing inert gas into a combustion chamber to cause combustion.
2. Description of the Related Art
Recently, in an internal combustion engine, for example, a diesel engine, to restrict generation of nitrogen oxides NOx, the diesel engine is structured such that an engine exhaust passage and an engine intake passage are connected by an exhaust gas recirculation passage (hereinafter, refer to an EGR) for recirculating an exhaust gas, that is, an EGR gas, into the engine intake passage via the EGR passage. In this case, since the EGR gas has a relatively high specific heat and can absorb a large amount of heat, a combustion temperature within the combustion chamber is lowered as an amount of the EGR gas is increased, that is, a rate of the EGR (EGR gas amount/(EGR gas amount+intake air amount)) is increased. When the combustion temperature is lowered, a generated amount of nitrogen oxides NOx is lowered. Therefore, the more the EGR rate is increased, the lower the generated amount of nitrogen oxides NOx becomes.
As mentioned above, it has been conventionally known that the generated amount of nitrogen oxides NOx is lowered when the EGR rate is increased. However, in the case where the EGR rate is increased, a generated amount of soot, that is, smoke, suddenly starts increasing when the EGR rate exceeds a certain limit. With respect to this point, it has been conventionally considered that the smoke is unlimitedly increased when the EGR rate is increased beyond a certain limit. That is, it has been considered that the EGR rate at which the smoke suddenly starts increasing is the maximum allowable limit of the EGR rate. Accordingly, the EGR rate is defined within a range which does not exceed the maximum allowable limit. The maximum allowable limit of the EGR rate is significantly different in relation to the type of engine and fuel, however, is within a range between approximately 30% and 50%. Therefore, in the diesel engine, the EGR rate is restricted to the range between 30% and 50% at the maximum.
As mentioned above, since it has been conventionally considered that the maximum allowable limit exists with respect to the EGR rate, the EGR rate has been defined within a range which does not exceed the maximum allowable limit. Therefore, the generated amount of nitrogen oxides NOx becomes as small as possible. However, even when the EGR rate is defined as discussed above, a reduction in the generated amount of nitrogen oxides NOx has a limit such that in actuality a significant amount of nitrogen oxides NOx is still generated.